Charles Reisman, MS; Atsushi Kubota, MS; Masahiro Akiba, PhD; Catharine Chisholm, PhD; and Michael J. Sinai, PhD
Conventional optical coherence tomography (OCT) has dramatically changed clinical practice over the last 15 years and is now an indispensable tool for clinicians in the detection and management of most ocular diseases. The rapid rise and integration of OCT technology into clinics around the world is principally due to the high-resolution cross-sectional images and thickness maps that provide critical structural information of the retina and choroid that cannot otherwise be visualized or appreciated. A new major advancement in OCT technology now enables the same OCT devices to image the retinal and choroidal microvasculature with extremely fine detail. Previously, this could be performed only with fluorescein angiography (FA) or indocyanine green angiography; however, now OCT devices can image the same vasculature without dyes or injections, using the same noninvasive scanning methods as in conventional OCT. In fact, the microvasculature imaged by optical coherence tomography angiography (OCTA) may reveal even finer detail than FA as well as allow visualization over specific depth-resolved tissue layers. It is likely that OCTA will gain a prominent role in clinical imaging and may even become an important everyday tool similar to conventional OCT.
OCTA is performed using the same device as conventional OCT. Instead of detecting the intensity of reflected light in depth as conventional OCT does, however, OCTA detects changes in the reflected light over time. In a truly static eye, any change in intensity at a particular point occurring over a short period of time is due to moving red blood cells within the vasculature. By detecting the areas of motion contrast, the microvascular network can be mapped and visualized. Furthermore, because OCT produces image data over a multitude of depths in tissue, OCTA images are also inherently depth resolved, making it possible to distinguish blood flow in one tissue layer from that in another. Of course, there are obstacles that must be overcome such as head and eye movements and background noise, and specific algorithms are required to detect motion contrast. Each company that offers OCTA solves these issues differently. This chapter will describe the Topcon method used in the Topcon Swept Source OCT device, the DRI Triton, and will include example images from normal and diseased eyes.
TOPCON OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY METHOD
OCTA methods are generally based on the measurement of motion contrast. In a practical implementation, ocular B-scan data may be scanned 2 or more times in the same location, and a calculation is performed across corresponding pixels in each frame or combination of frames in order to quantify the degree of motion contrast. This measure is then presumed to correspond to angiographic flow, as blood flow is the primary cause of signal change under normal imaging conditions after bulk motion has been accounted for. Some angiographic methods compute the differences between image frames, whereas others may compute the variance over an arbitrary number of frames.
Figure 10-1. En face OCTA of the macula using the DRI Triton and Topcon IMAGEnet6 software showing superficial (top left) and deep (top center left) capillary plexus as well as the outer retina (top center right) and choriocapillaris (top right) using default settings. (Bottom left) Corresponding OCT cross-section, (bottom center) color composite microvasculature, and (bottom right) en face OCT superimposed on the color fundus photo are also shown.
The Topcon method for OCTA is called OCTARA, which stands for OCTA Ratio Analysis. This name describes the basic process Topcon uses to detect the retinal and choroidal microvascular pattern. For OCTA processing with OCTARA, B-scans are captured 4 times at each scan location and are registered to each other. Motion contrast data are generated by computing a ratio-based result, r, between corresponding image pixels: where I(x,y) is the OCT signal intensity of a particular pixel, N is the number of scanned B-scan combinations at the given location, and i and j represent the 2 frames within any given frame combination. This formula represents a relative measurement of OCT signal amplitude change that optimizes angiographic visualization over both the retina and choroid, and also enhances the minimum detectable signal relative to amplitude decorrelation. It should be noted that the directionality of the ratio is arbitrary (ie, numerator vs denominator) and that the subtraction from unity is an optional operation that serves to conveniently orient the direction of the display range similarly to other calculation methods such as differentiation and decorrelation. The following formula is used.
r = ratio result at each pixel (x,y)
N = total number of B-scans compared (typically 4)
I = intensity value at each pixel (x,y)
i and j = 2 frames (B-scans) within any given set of 4
B-scans are then segmented in the usual way and this segmentation is used to create layer-specific en face OCTA images (see the following for examples).
At the time of this publication, Topcon’s OCTARA method does not have Food and Drug Administration clearance, and is not yet commercially available in the United States. It is available in countries outside the United States. Please check with the local Topcon representative to know if it is available in a specific country.
OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY EXAMPLES IN HEALTHY EYES
Figure 10-1 is a screenshot from the IMAGENet6 software (Topcon) used to visualize the results of the OCTA scans (this is the standard software for the DRI Triton). In Figure 10-1, the foveal region, the superficial retinal vasculature, and the foveal avascular zone are readily apparent. The inner vascular plexus in the ganglion cell layer and an outer layer of capillaries in the inner nuclear layer (INL) are readily distinguishable as well. The densely packed choriocapillaris network is also visualized. Four key layers in depth are displayed starting with more anterior retinal surfaces and moving downward in depth. In addition to the layer-specific OCTA images, the software also displays the corresponding B-scan with segmentation for that layer and also a color-coded composite image of the entire microvasculature (all layers combined and color-coded for depth; see bottom center panel for Figure 10-1). The software also allows the user to manually adjust the depth and the width of the band for visualization of the microvasculature at any specific region desired.
Figure 10-2. En face OCTA of the optic nerve head using Topcon IMAGEnet6 software showing the 4 default depth areas for visualization. The top left OCTA image includes the vitreous and extends 49.4 μ below the inner limiting membrane (ILM) (see bottom left panel), the middle left OCTA image shows the surface region down to 70.2 μ below the ILM, the middle right OCTA image superficial vascular shows the surface region down to 130 μ below the ILM, and the bottom right panel shows the deepest layers from the ILM + 130 μ down to the bottom of the scan. The bottom row of B-scans shows the boundaries selected by the software by default for visualization of each microvascular region in depth. (Reprinted with permission from Topcon.)
For the macula, the superficial layer default segmentation is defined as the region between the inner limiting membrane (ILM) + 2.6 μ to the inner plexiform layer (IPL)/INL border + 15.6 μ. The deep layer default segmentation goes from the IPL/INL border + 15.6 μ to the IPL/INL border + 70.2 μ. The outer retina default segmentation goes from the IPL/INL border + 70.2 μ down to Bruch’s membrane. The choriocapillaris default segmentation is from Bruch’s membrane down to 10.4 μ. These default settings were chosen to best visualize the key regions of the microvasculature (and absence of microvasculature as in the case of the outer-retinal region). The user also has the flexibility to visualize any depth by manually customizing the specific depth and width of the OCT band.
In the optic nerve head region, the radial peripapillary capillary network and microcirculation in the optic disc can be visualized (Figure 10-2). As with the macular scans, 4 default depth regions are chosen and displayed beginning at the more anterior superficial layers and moving downward. As with the macula, however, the user can also manually select the depth and width of the band for optic disc OCTA scans in order to better visualize specific regions of interest.
Another example of a healthy eye is shown in Figure 10-3. Here, the macular scan and the optic disc scan are both shown along with the 4 default regions for each scan. The middle panel shows the B-scans through the fovea for the macular scans along with the segmentation boundaries for each corresponding region in depth. The outer retinal displays can optionally incorporate a subtraction of inner retinal signal in order to reduce the occurrence of projection artifacts.
Figure 10-3. Vascular patterns at various depths in a normal eye for the macula (top) and optic disc (bottom). The middle section shows the B-scans and the segmentation boundaries used for the macula optical coherence tomography angiography images.
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